1. Field of the Invention
The present invention relates in general to a process for forming resistors on a printed circuit board. More particularly, it relates to a process by which polymer thick film resistors and metal thin film resistors can be successively produced on the same layer of a printed circuit substrate.
2. Description of the Related Arts
The density of components and conductor circuits placed on printed circuit boards has been increasing in response to the need to accommodate increasingly complex electronic circuitry in smaller spaces. In addition to accommodating a greater number of conductor paths than heretofore, many printed circuit boards being manufactured today have a greater number of discrete components mounted thereon. Frequently, electronic circuits contained on printed circuit boards require a very large number of resistors to perform various functions, for example, terminating circuits. Heretofore, in most instances such terminations were required to be made by mounting resistors on the surface of a circuit board and connecting the resistors to conductors through holes which passed from the board's surface through one or more conductor planes to the conductor paths to which the resistors are to be connected- Resistors used to accomplish such terminating function could be either conventional carbon composition resistors or resistors screened onto the circuit board's top surface using polymer thick film (PTF) resistive pastes. In the event that surface mounting of the resistors is not suitable, e.g. where resistive terminations are required in a board having many layers and limited surface area, the circuit designer could use a material sold under the name "Ohmega-Ply" by Ohmega Technologies, Inc. The Ohmega-Ply material is composed of a layer of resistive material sandwiched between upper copper conductive layers and lower core material (PR-4 or Polyimide). The Ohmega-Ply material may be used to make resistor-conductor networks using a photolithographic process. The Ohmega-Ply material may be etched using conventional etching processes to produce circuit layers having integral discrete resistors. However, one problem with Ohmega-Ply material which makes it undesirable for use in multilayer printed circuit boards is that it is difficult to produce resistors for a layer whose values substantially differ from each other within the layer, e.g., vary by more than one decade.
U.S. Pat. No. 4,021,277 describes a method of forming a thin resistor film which comprises: depositing by cosputtering a metastable alloy film on a substrate, annealing said resistor film, and determining the geometry of The film by a normal photo-etch process. U.S. Pat. No. 4,368,252 describes a method for manufacturing a printed circuit board with resistance elements, in which a resistor pattern film and a conductor pattern film are formed on the two surfaces of a high conductive material layer such as a copper foil., respectively, and an insulating support is combined with the resistor pattern film on the high conductive material layer. Methods for forming metallic thin film resistors arc described in U.S. Pat. Nos. 4,540,463, 5,039,570, and 4,865,573. One major drawback with these methods is the incapability of producing resistors of high resistivity. Another method reading to the general subject matter is described in U.S. Pat. No. 4,870,746. The method includes the steps of manufacturing individual layers of the multilayer board with electrical circuits thereon and subsequently screening on one or more values of resistors using a polymer thick film resistive ink. The individual layers are thereafter bonded together into a multilayer board. The forgoing method has the disadvantage that the geometry and thickness of the resistors is difficult to control, resulting in variations in the resistivities.